1. Field of the Invention
This invention relates to a dressing method of a polishing pad used in CMP (Chemical Mechanical Polishing) and apparatus designed for such a method, specifically to a detection method of an endpoint of dressing and an apparatus implementing the detection method.
2. Description of the Related Art
The CMP has been known as a polishing technology used in planarization of a semiconductor wafer. The CMP is a polishing method using a slurry of abrasives and chemical solution in order to avoid damage to the wafer due to mechanical polishing.
A wafer is polished in CMP by rotating a polishing table with a polishing pad mounted on it and rotating the wafer while pressing the wafer to the polishing pad.
As the number of wafers polished increases, it becomes increasingly difficult for the polishing pad to hold the abrasives on it, because projections and depressions on a surface of the polishing pad decrease and polishing debris goes into the projections and depressions. As a result, the polishing rate in polishing the next wafer is reduced, leading to deterioration in uniformity of a surface of the wafer.
Thus, a dressing is applied to the polishing pad in order to recover the projections and depressions on the surface of the polishing pad to a predetermined roughness. Dressing is performed by rotating the polishing table with the polishing pad mounted on it and rotating a dresser having abrasive grains of diamond while pressing the dresser to the polishing pad. The dressing is used to be performed longer than the minimum time necessary to regenerate the projections and depressions on the surface of the polishing pad in order to avoid insufficient dressing. Applying such excessive dressing has made the life of the polishing pad shorter than expected.
In order to avoid excessive dressing, an optimum endpoint of the dressing has been determined by monitoring the surface conditions of the polishing pad.
Some methods to monitor the surface of the polishing pad are described below, for example. One method is contact type surface displacement measurement. This measurement is performed by touching the surface of the polishing pad by a contact sensor capable of detecting the projections and depressions on the surface of the polishing pad. Another method is a destructive inspection performed by cutting a portion of the polishing pad. In the destructive inspection, a surface condition of the cut-out portion of the polishing pad is inspected with a SEM (Scanning Electron Microscope) or the like.
Further details may be found in Japanese patent No. 2851839 and Japanese Patent Application Publication No. 2003-100683.
In the conventional contact type surface displacement measurement of the polishing pad, however, there is a problem that the surface of the polishing pad is damaged. Also, with the destructive inspection performed by cutting a portion of the polishing pad and inspecting it with SEM, the need for replacing the polishing pad with new one after the inspection increases a cost of dressing and consumes time to replace the polishing pad.
Thus, this invention is made to offer a method to quantitatively detect an optimum endpoint of dressing with non-destructive monitoring of the polishing pad.